Thin Film Microsensors Research Articles

Revealing the dynamic temperature of the cathode catalyst layer inside proton exchange membrane fuel cell by experimental measurements and numerical analysis

Temperature variation inside the cathode catalyst layer (CCL) is significant, which could greatly damage fuel cell performance and durability. However, limited by CCL microstructure and complex physicochemical processes, the knowledge regarding CCL dynamic temperature is indeed insufficient. Thus, this work integrates a pre-prepared thin-film microsensor into the fuel cell to measure CCL surface temperature under dynamic load. At the same time, an improved transient thermal model considering local mass transport and liquid effect on thermal contact resistance (TCR) is established for the first time to analyze the internal process. Results show that CCL temperature is greatly affected by local resistance and interface accumulated water. And, as confirmed by both experiment and simulation, the CCL surface exhibits 0–7 °C temperature variation, whose profile changes sharply at the beginning and then evolutes slowly, eventually taking tens or hundreds of seconds to reach a new steady state due to thermal inertia. Moreover, the temperature is further increased by ∼1 °C as cathode humidity increases or stoichiometric ratio decreases, but only slight changes with the anode condition. The significant role of cathode operation is attributed to the greater mass transfer resistance and more sensitive intrinsic oxygen reduction reaction to reactant change. This study is expected to provide a deeper understanding of CCL dynamic temperature, thus improving fuel cell thermal management.

Structural design and optimization of a turning tool embedded with thin-film strain sensors for in-process cutting force measurement

A tool system integrated with machining and measurement is constructed by using thin film strain sensors and tool integration technology, which provides a new research method for studying the machining mechanism and automatic monitoring of the machining process. In this paper, three kinds—elastic sleeve type, fast insert type, and insert type—of tool cutting force measurement systems of the turning tool embedded with thin-film micro-sensors are designed. High strain sensitivity can be obtained through setting the elastic region of the tool bar on the premise of meeting the tool strength and stiffness. The system can realize integration between the thin-film strain sensor and tool bar quickly and efficiently. It is simplified as a cantilever beam structure for analysis after setting the shape of the sensor elastic substrate as a rectangle. The expression between strain and deflection of the elastic substrate is established when the free end is subjected to tangential force and bending moment, and the influence of the substrate’s structural parameters on the deflection is analyzed. In order to further improve the strain effect, six kinds of other shapes of the substrate structure are optimized, deducing the expression between the structure parameters and deflection; the corresponding substrate structure atlas is established, comparing and analyzing the deflection of the rectangular structure substrate. Five kinds of metal substrate materials are selected. In the functional film system of the thin-film strain sensor, the scheme of aluminum oxide and titanium nitride as the transition layer, silicon nitride as the insulating layer, and Ni–Cr alloy as the resistance grid layer is adopted. The research results show that this scheme can improve the adhesion between the insulating layer and the substrate, improve the resistance sensitivity coefficient of the sensor, and meet the application requirements of the cutting force measurement system.

Feasibility study of a partially released lead-zirconate-titanate (PZT) Diaphragm micro-actuator

Piezoelectric thin-film micro-sensors and actuators often appear in the form of a diaphragm anchored around its entire boundary. When a micro-device is scaled down in size, its sensitivity reduces and natural frequency increases, drastically lowering its performance. The purpose of the paper is to study the feasibility of introducing through-etched slots to partially release the diaphragm at its anchor. As a result, a micro-device can be scaled down in size without significantly altering its sensitivity and natural frequency. In this paper, we first present a finite element simulation proving the concept. We then describe processing steps to fabricate a lead-zirconate-titanate (PZT) thin-film diaphragm sensor/actuator with a partially released boundary. Challenges encountered in the fabrications, such as cat ear and electrode non-uniformity, are explained and overcome. As a case study, we demonstrate the feasibility to design, fabricate, and test an intra-cochlear micro-actuator probe that employs three partially released PZT diaphragms at the tip of a cantilever. Experimental measurements indicate that the sensitivity is dominated by the diaphragm deflection, while the first natural frequency is dominated by the cantilever structure. Finite element simulations not only confirm the experimental measurements but also optimize diaphragm dimensions for sensor/actuator performance.

Microfabrication and Characterization of Tool Embedded Ni-Chrome Thin Film Micro-Sensors for Cutting Force Measurement

This paper investigated the fabrication and design of embedded Ni-chrome thin-film strain gauges as micro-sensors in tool holders to measure the cutting force in machining operations. A Ni-chrome thin film as piezoresistive material sensor device is embedded within a substrate structure through brazing bonding process, which consists of a Ti6Al4V substrate, a Nickel-chromium thin film sensor and an Alumina insulating layer. The thin-films were characterized by 3D Super Depth Digital Microscope, SEM/EDS, Stylus profiler, to study microstructure, material composition, thickness and sheet resistance respectively. The thin-film strain gauges are calibrated in a cantilever beam setup. Accordingly, in-process cutting force measurement systems are established. The results showed that the thin-film sensor had good linearity and more elaborate structure and superior properties.

Transient Temperature and Heat Flux Measurement in Ultrasonic Joining of Battery Tabs Using Thin-Film Microsensors

Process physics understanding, real time monitoring, and control of various manufacturing processes, such as battery manufacturing, are crucial for product quality assurance. While ultrasonic welding has been used for joining batteries in electric vehicles (EVs), the welding physics, and process attributes, such as the heat generation and heat flow during the joining process, is still not well understood leading to time-consuming trial-and-error based process optimization. This study is to investigate thermal phenomena (i.e., transient temperature and heat flux) by using micro thin-film thermocouples (TFTC) and thin-film thermopile (TFTP) arrays (referred to as microsensors in this paper) at the very vicinity of the ultrasonic welding spot during joining of three-layered battery tabs and Cu buss bars (i.e., battery interconnect) as in General Motors's (GM) Chevy Volt. Microsensors were first fabricated on the buss bars. A series of experiments were then conducted to investigate the dynamic heat generation during the welding process. Experimental results showed that TFTCs enabled the sensing of transient temperatures with much higher spatial and temporal resolutions than conventional thermocouples. It was further found that the TFTPs were more sensitive to the transient heat generation process during welding than TFTCs. More significantly, the heat flux change rate was found to be able to provide better insight for the process. It provided evidence indicating that the ultrasonic welding process involves three distinct stages, i.e., friction heating, plastic work, and diffusion bonding stages. The heat flux change rate thus has significant potential to identify the in-situ welding quality, in the context of welding process monitoring, and control of ultrasonic welding process. The weld samples were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to study the material interactions at the bonding interface as a function of weld time and have successfully validated the proposed three-stage welding theory.

Tool Embedded Thin Film Microsensors for Monitoring Thermal Phenomena at Tool-Workpiece Interface During Machining

Existing thermal sensors for machining processes are having difficulties to reliably provide high spatial and temporal resolutions for monitoring and control of the dynamic thermal phenomena at the tool-workpiece interface during machining. This paper presents a novel approach to obtain transient tool internal temperature data reliably from a very close distance to the tool-workpiece interface. An array of nine microthin film thermocouples, fabricated using standard microfabrication methods, has been embedded into polycrystalline cubic boron nitride (PCBN) cutting inserts by means of diffusion bonding. Scanning electron microscopy was performed to examine thin film sensor and host material interactions at the bonding interface and to determine optimal bonding parameters. The thin film microthermal sensors were statically and dynamically characterized by heating in a tube furnace and application of pulsed laser radiation. The embedded thin film thermal sensors exhibit good linearity, sensitivity, and extremely fast response time. The instrumented PCBN inserts were applied in machining of aluminum alloy 6061 disks at various cutting speeds and feed rates. Embedded into the PCBN inserts at numerous distances of 75–450 μm from tool rake face and 100–500 μm from tool flank face, these microsensors enabled the sensing of transient cutting temperature fields with high spatial and temporal resolutions. The temperature data obtained during cutting demonstrate the functionality of the tool embedded microthermal sensors and their value for fast, accurate, and reliable monitoring, which will be of significance for both fundamental study and in-process control of machining operations.

Accurate Temperature Monitoring in Laser-Assisted Polymer Bonding for MEMS Packaging Using an Embedded Microsensor Array

This paper presents an experimental study of an accurate temperature-monitoring method using an embedded thin-film microsensor array for laser-assisted polymer bonding for MEMS packaging. The work is carried out using a fiber-coupled diode laser system and benzocyclobutene polymer as the bonding material. Beam-forming optical elements are used to generate top-hat and frame-shaped beam profiles. Platinum-based sensor arrays are fabricated using sputtering and ion-beam etching methods. Peripheral sensors are embedded at the interface between the polymer sealing ring on the top (capping) substrate and the sensor substrate in the bonding process. The embedded peripheral sensors allow precise monitoring of the temperature profile of the polymer track in the laser-assisted thermal curing process for substrate bonding. The sensor at the center of the array can monitor the temperature that would be experienced by a MEMS device in a manufacturing environment. Results show that accurate temperature monitoring can be obtained using the embedded sensor array. A lower temperature than that required for bonding is seen at the center of the bottom (device) substrate. This is a highly desirable effect for packaging of temperature-sensitive devices. In addition, the effects of substrate material and arrangement of heat dissipation on the resultant temperature profiles have been investigated.

Micro gap effect on dilute H2S sensing properties of SnO2 thin film microsensors

The micro gap effect has been investigated on dilute H2S sensing using SnO2 thin film microsensors equipped with Au or Pt micro gap electrode. The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB) and the gap size was varied in the range of 0.1–1 μm. For the SnO2 sensors equipped with Au micro gap electrode, the sensor response to dilute H2S was slightly increased with decreasing gap size, suggesting the small micro gap effect. The sensor response was divided into that at oxide–electrode interface (Si) and at oxide grain boundary (Sgb). It was found that the small Sgb/Si ratio was responsible for the small micro gap effect and that the high sensor response at grain boundary was characteristic in SnO2–H2S system. On the other hand, the SnO2 sensors with Pt micro gap electrode, the clear and large micro gap effect was obtained on dilute H2S sensing. In the Pt electrode sensor, the large Sgb/Si value induced the large micro gap effect. It was found that the Pt/SnO2 interface showed much higher sensor response than the Au/SnO2 interface. The electrode material strongly affected the sensing properties of semiconductor gas sensor with micro gap electrode.

Development of a long-term stable organic membrane-based thin-film microsensor using new-type substrate surface treatment

A novel long lifetime organic membrane-based thin-film Cu(II) microsensor has been elaborated. The advantages of the suggested microsensor include: excellent stability, long lifetime, and reasonable good selectivity. The significant improvements of the lifetime (>4 months) of the organic membrane-based thin-film microsensors have been realized for the first time using new-type electrodeposition treatment of the solid-state substrate surface in combination with a new nebulization method for applying the organic membrane coating mixture on the thin-film gold substrate. The electrochemical behavior of the thin-film microsensor in terms of ionic sensitivity, limit of detection, the effect of the pH, dynamic response time, Nernstian response interval, selectivity coefficients and lifetime has been evaluated. The results are compared to those obtained with the conventional ion-selective and coated graphite rod macroelectrodes prepared with the same ionophore. The reliability of the suggested thin-film microsensor with the low cost of its microfabrication makes it promising for the miniaturized application.

Novel Organic Membrane-based Thin-film Microsensors for the Determination of Heavy Metal Cations

A first step towards the fabrication and electrochemical evaluation of thin-filmmicrosensors based on organic PVC membranes for the determination of Hg(II), Cd(II),Pb(II) and Cu(II) ions in solutions has been realised. The membrane-coating mixture used inthe preparation of this new type of microsensors is incorporating PVC as supporting matrix,o-nitrophenyloctylether (o-NPOE) as solvent mediator and a recently synthesizedHg[dimethylglyoxime(phene)]2+ and Bis-(4-hydroxyacetophenone)-ethylenediamine aselectroactive materials for Hg(II) and Cd(II), respectively. A set of three commercialisedionophores for Cd(II), Pb(II) and Cu(II) has been also used for comparison. Thin-filmmicrosensors based on these membranes showed a Nernstian response of slope(26-30 mV/dec.) for the respective tested cations. The potentiometric responsecharacteristics (linear range, pH range, detection limit and response time) are comparablewith those obtained by conventional membranes as well as coated wire electrodes preparedfrom the same membrane. The realisation of the new organic membrane-based thin-filmmicrosensors overcomes the problem of an insufficient selectivity of solid-state-based thin-film sensors.

Fabrication and Characterization of a Poly (3-Hexylthiophene) Thin Film Micro-sensor for Hypergolic Vapor Detection

ABSTRACTA passive chemiresistor micro-sensor was investigated for the detection of hydrazine compounds. Hydrazine compounds are a highly toxic and carcinogenic species exhibiting toxic effects in humans at very low levels of exposure. Therefore, a sensor capable of detecting ppb levels of hydrazine compounds is required to insure the safety of personnel. The present study describes the fabrication, testing, and characterization of a low-cost, ultrasensitive Poly (3-Hexylthiophene) (P3HT) thin film-based micro-sensor for the detection of hydrazine compounds. Standard microelectronic manufacturing techniques were used to form a micro-sensor composed of a silicon substrate, interdigitated gold electrodes, and P3HT sensing film. Responses of the micro-sensor to hydrazine compounds at different temperatures and concentration levels are reported. When exposed to 25 ppm hydrazine in nitrogen, the sensor's resistance was measured to change from a few ohms to over 10 Megaohms. The thermal stability of the P3HT micro-sensor and the method to improve thermal stability are also explored. Thermally annealing the P3HT micro-sensor was found to improve thermal stability at high temperatures. Moreover, the sensor exhibits good specificity to hydrazine and does not respond to the presence of NO2 and/or N2O.

Effect of micro-gap electrode on sensing properties to dilute chlorine gas of indium oxide thin film microsensors

Effects of gap size on dilute Cl2 gas sensing properties have been investigated for the In2O3 thin film microsensors equipped with micro-gap electrode. The micro-gap electrodes with various gap sizes (0.1–2μm) were fabricated on SiO2/Si substrate by means of MEMS techniques (photolithography and FIB). Then the In2O3 thin film was deposited on the micro-gap electrode to be micro gas sensor. The resistance of In2O3 microsensor was increased upon exposure to dilute Cl2 gas of ppm-level, while the sensor responded to Cl2 gas less than 0.5ppm with resistance decrease. Interestingly, in both cases the sensitivities were increased with decreasing gap size, although the starting gap size of sensitivity increase were different from each other. The high sensitivity was obtained for In2O3 microsensor with 0.1μm gap. The number of grains was decreased with decreasing gap size, inducing the increasing contribution of interface between electrode and oxide. The sensitivity was increased due to the high sensitivity at interface. The similar effect of number of grains in the micro-gap was examined for the grains with different size packed in the 0.1μm gap electrode. The different grain size was obtained after calcinations at 600–850°C. With decreasing number of grains, the sensitivity to 1ppm Cl2 (resistance increase) was increased, while the sensitivity to 0.5ppm Cl2 (resistance decrease) was decreased. It is assumed that the condition of oxide–electrode interface or In2O3 surface is changed with calcination.

Effect of micro-gap electrode on detection of dilute NO 2 using WO 3 thin film microsensors

The micro-gap electrodes with various gap sizes (0.1–1.5 μm) were fabricated on Si substrate by using MEMS techniques (photolithography and FIB) and the WO 3 thin film was deposited on the micro-gap by suspension dropping to be micro-gas sensor. The effect of gap size on sensing properties to dilute NO 2 (0.05–0.5 ppm) was investigated at 200 °C. The sensitivity to dilute NO 2 almost unchanged irrespective of gap size larger than 0.8 μm. On the other hand, the sensitivity tended to increase with decreasing gap size less than 0.8 μm. The sensitivity ( S = R g/ R a) to 0.5 ppm NO 2 was as high as 57 at gap size of 0.11 μm and was expected to increase further when the gap size was decreased less than 0.1 μm. The behavior was explained with the number of WO 3 grains in the micro-gap and the resistance changes at two kinds of boundaries (grain boundary and electrode–grain interface).

Development of multisensor systems based on chalcogenide thin film chemical sensors for the simultaneous multicomponent analysis of metal ions in complex solutions

A new type of thin film chemical microsensors based on chalcogenide glass-sensitive materials was developed by means of silicon planar technology and pulsed laser deposition technique. These miniaturised ion-selective electrodes (ISEs) exhibit Nernstian responses over five concentration decades with detection limits of 1×10 −7 mol/l towards the primary ions Cu and Pb, and 4×10 −7 and 3×10 −5 mol/l towards Cd and Tl, respectively. The thin film microsensors have been shown to be perspective instruments for the simultaneous multicomponent analysis of complex liquid media based on the principles of an ‘electronic tongue’ device. Incorporating the thin film sensors into a sensor array allowed the multicomponent analysis of heavy metal-ion species (Pb 2+, Cd 2+, Zn 2+ and Fe 3+). The concentrations of Pb 2+-, Cd 2+- and Zn 2+-ions can be determined simultaneously by direct potentiometric measurements using a sensor array of seven all-solid-state thin film chemical microsensors with an accuracy of 15–30%. The sensor array allows overcoming the problem of an insufficient selectivity of single sensors. The suggested microsystem-compatible fabrication technique favours a further miniaturisation, aimed to a fully integrated electrochemical microsystem.

Microsensor for the measurement of water content in the human skin

We have developed a thin film microsensor based on interdigital electrodes deposited on a glass substrate. If the film side of the sensor chip is pressed onto human skin, the change of capacitance can be used as a measure for the water content of the skin. As the front side of the sensor must remain flat, the interconnection wires have been fixed at the backside of the chip. In this paper we report on measurements with a frequency of 100 kHz to 2 MHz and a voltage of 100 mV. We compare measuring results on normal skin, atopic (dry) skin and skin moisturized by a humectant.

A new multi-function thin-film microsensor based onBa1-xLaxTiO3

A new multi-function microsensor with a barium lanthanum titanate(Ba1-xLaxTiO3) thinfilm is developed for the fabrication of a thin-film resistor and a metal-insulator-semiconductor(MIS) capacitor on a SiO2/Si substrate. The results reveal that thethin-film resistor has good sensitivity to visible light and good thermalsensitivity within the temperature range of 28-400 °C. The MIS capacitor issensitive to changes in the relative humidity (RH), with a130% change of the current for a humidity change from 12 to 92% RH at a test frequencyof 500 Hz, and with short response times of 5 and 4.5 s for the adsorption anddesorption of water vapor, respectively. The effects of the test frequency on the photo,thermal and humidity sensitivity performances are also investigated. Since thelifetime of charge carriers of Ba1-xLaxTiO3 thin films is only 16 ms,about half that of Sr1-xLaxTiO3 thin films, the device can detect lightmodulation at higher frequencies.

Micromachined thin film solid state electrochemical CO 2, NO 2 and SO 2 gas sensors

Among the many physical and chemical strategies used to make air pollution gas sensors for CO x , NO x , SO x monitoring, those employing electrochemical detection means offer the highest selectivity, long lifetimes, low drift and low costs of manufacture. Targeting the three gases CO 2, NO 2 and SO 2, new solid-electrolyte-based, selective potentiometric gas sensors were fabricated in both bulk and thin film versions. For all six bulk and thin film sensors, the EMF responses were related to the analyzed target gas pressure by the Nernst law. For each of the target gases CO 2, NO 2 and SO 2, our thin film micro-sensor exhibits excellent gas-sensing sensitivity, response time, selectivity and recovery compared to the bulk sensor. The carbon dioxide micro-sensor characteristics obtained at 250°C were 45±3 mV/decade for the sensitivity, 10 to 30 s for the response time and 60 s for recovery time. The nitrogen dioxide micro-sensor characteristics obtained at 200°C were 48±3 mV/decade for the sensitivity, 2 to 10 s for the response time and 10 s for recovery time. The sulfur dioxide micro-sensor characteristics obtained at 250°C were 50±3 mV/decade for the sensitivity, 1 to 2 s for the response time and 5 s for recovery time. In comparison, the bulk sensor CO 2 characteristics obtained at 250°C were 50±3 mV/decade for the sensitivity, 60 to 300 s for the response time and 900 s for recovery time. To verify selectivity in air samples, the sensitivity of each gas detector with respect to the gases CO 2, NO 2 and SO 2, oxygen, CO, and humidity were evaluated. The CO 2 sensor was insensitive to the later three gases, but there was a measurable EMF drift produced for NO 2 at concentrations above 10 ppm and for SO 2 at concentrations above 80 ppm. Similarly, the NO 2 sensor operating at 200°C was insensitive to all other gases except for SO 2 at concentrations above 80 ppm. Finally, the SO 2 sensor operating at 250°C was insensitive to all other. We optimized our processing to obtain a dense strongly adhering deposited solid electrolyte thin film. XPS and XRD studies show that the chemical composition and structure of the deposited solid electrolyte thin-film and targets for RF magnetron sputtering were the same to within experimental error. For brevity, complete results are presented only for the CO 2 micro-sensor. The results for the other two gas micro-sensors are essentially the same and will be published elsewhere.